A 33-year-old Indian-origin police officer in the US state of California has been killed after being shot by an "armed" unidentified gunman while he was conducting a traffic stop.

Corporal Ronil singh of the Newman Police Department was shot and killed during a traffic stop when he was "working overtime on Christmas night to provide the best for his family."

"A few moments later he called out 'shots fired' over the radio," the Stanislaus County Sheriff's Department said in a statement, which is leading the investigation.

"Multiple agencies responded to assist, and Singh was found at the scene with gunshot wounds."

He was transported to a local hospital where he was pronounced dead, the department said.

It said the suspect had fled the scene in his vehicle before the police arrived. The department is leading the investigation and have released surveillance photographs of the suspect and vehicle, asking for any information on the identity of the suspect.

The California Highway Patrol (CHP) described him as Hispanic.

"Suspect is considered armed and dangerous and may be in possession of a firearm," the CHP said.

Singh was a more than 7-year veteran of the Newman Police Department and was assigned as a canine officer. Prior to joining the Newman Police Department, he served with the Merced County Sheriff’s Department.

Corporal Singh is survived by his wife, Anamika, and 5-month son.

A report in KCRA3 news said that Singh was a native of Fiji and had immigrated to America. It quoted Stanislaus County Sheriff's Department spokesman Raj Singh as saying that he knew the slain officer.

"He was living the American Dream. He immigrated here from the Fiji Islands, just like my parents did, and was definitely enjoying the American Dream," Deputy Raj Singh said.

Tributes and condolence messages for Singh poured from police departments and officials across the country.

California Governor Edmund Brown extended his condolences to Singh's wife, their young son and colleagues.

"Our hearts are with the entire community of Newman and law enforcement officers across the state who risk their lives every day to protect and serve the people of California," Brown said.

In honour of Singh, Capitol flags in California will be flown at half-staff, the Governor's office said.

The New York City Police Department (NYPD) Commissioner James P O'Neill tweeted that the entire New York Police Department is "thinking of the family, friends and colleagues of Corporal Ronil Singh… of the Newman Police Department (CA), who was murdered early this morning while conducting a traffic stop — just hours after taking Christmas photos with his wife and son. #NeverForget."

The New York Police Department, in a separate tweet, said after years of service with the Newman Police Department, "Singh was shot & killed while conducting a traffic stop shortly after Christmas day. We mourn the loss of this hero and send our condolences to his friends, family, and colleagues."

The Indian Officers' Society at the NYPD expressed sadness at Singh's death, tweeting that Singh was killed in the line of duty and recalled his service to the community.

Newman Police Chief Randy Richardson said the Newman Police family is devastated by Singh’s death.